I guess you could think of it in percentages. So, if you went through about a third of a quart (a little over 10 ounces), one would lose about 10 ounces of oil per oil change, that’s if you changed your oil every 2500 - 3000 miles and oil loss remained linear.
Three quarts = 96 ounces. You lose approx 10% of your oil capacity per oil change. No big deal I guess.
Now, I would think that if one was losing 8 ounces per 1000 miles; that seems like a bit much. If the loss remained linear, you would lose 24 ounces in a 3000 mile service interval. If the loss remained constant, one would see a loss in total oil capacity of 25% over 3000 miles.
I know when I change my oil using the oil scavenger; I get approximately 24 ounces or so of crap oil out before the oil looks clean. My guess is that there is about that much oil in the lines; pump, cam case and oil filter housing, etc.
Having said that, if I was losing 8 ounces every 1000 miles, I would not let it go too long. Figuring the numbers above, there would only be a little over a quart in the tank before a 3000 mile service. That doesn't leave much for heat transfer.